1. Field of the Invention
This invention pertains to safety systems for mobile carriages, and more particularly to apparatus that employs photoelectric energy to prevent unintended movement of mobile carriages.
2. Description of the Prior Art
High density mobile systems for storing books, files, and other media are well known. In mobile storage systems, one or more mobile carriages roll along rails embedded in a building floor. As a mobile carriage moves, it opens or closes an aisle between it and an adjacent carriage or a stationary member such as a wall or end cabinet. It is essential that a mobile carriage not move to close an aisle if a person or object is in the aisle.
Accordingly, it is also well known to equip high density mobile storage with safety devices that prevent unintended mobile carriage movement. In particular, safety systems that use steady state photoelectric energy have become increasingly popular. Typical examples of photoelectric safety systems may be seen in U.S. Pat. Nos. 5,121,975; 5,359,191; 5,427,444; and 5,670,778. Although not directly related to high density mobile storage, the substrate detector of U.S. Pat. No. 5,266,812 involves generally similar technology.
An undesirable characteristic of the safety systems of the foregoing patents is that each requires a large number of photoelectric transmitters for the systems to achieve their purposes. The transmitters must be spaced at some minimum interval along a mobile carriage. For long mobile carriages, therefore, the number of transmitters can be quite large. For example, if a one foot spacing is required between transmitters, and a mobile carriage is twenty feet long, twenty-one transmitters are necessary. The prior systems also invariably require a photoelectric receiver for each transmitter. Such systems are quite expensive. Further, the large number of transmitters requires considerable electric power to operate. The modern trend in electrical design is to minimize the power requirements for electrical equipment and accessories. It therefore is undesirable to include more transmitters than is necessary in high density mobile storage applications.
Our co-pending U.S. patent application Ser. No. 08/969,484 shows an exemplary safety system for high density mobile storage that requires only one photoelectric transmitter for an aisle. The single beam from the transmitter on a mobile carriage is reflected by a rotating mirror. The reflected beam cyclically sweeps the adjacent aisle and is detected by any of several photoelectric receivers on the other side of the aisle.
Despite recent advancements, continued development of safety systems for high density mobile storage is highly desirable.